This invention relates to a method and apparatus for producing flows of molecular gas atoms, in particular an atomic hydrogen flow. The invention is particularly useful in the manufacture, of semiconductor devices and integrated circuits.
The following is a list of references, which is intended for a better understanding of the background of the present invention.
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27. Ito M., Yamamato M., Nakamura S., Hattori T., xe2x80x9cPurification of diamond films by applying into the plasma stream in the cathode arc discharge plasma jet chemical vapor deposition techniquexe2x80x9d, J. Appl. Phys., 1995, 77(12), pp.6636-6640.
The manufacture of semiconductor devices and integrated circuits utilize the treatment of semiconductor structures in aqueous chemical solutions (the so-called xe2x80x9cwetxe2x80x9d methods) and in plasma of various gases (xe2x80x9cdryxe2x80x9d methods). Lately, there has been significant increase in the use of dry methods as compared to that of wet methods, and treatment in plasma is being replaced by treatment in xe2x80x9cremotexe2x80x9d plasma.
Dry treatments of semiconductor structures used in the industry utilize known sources of plasma and particles beams based on various configurations of radio-frequency (RF) discharge [2,3], microwave discharge under the condition of electron cyclotron resonance (ECR) [2, 4], glow and arc discharges of direct current [5,6].
A dry treatment technique based on the use of a flow of neutral kinetically enhanced chemically active particles (atoms, radicals and excited particles), and particularly, a flow of atomic hydrogen, has also been developed [1]. This technique is characterized by the minimal level of introduced defects and contaminations, and a high degree of the reproducibility and controllability of a treatment process, and is therefore considered as a perspective technology in the manufacture of semiconductor devices with critical dimensions less than 0.18 xcexcm. The successive realization of this technique and provision of high rates of treatment of semiconductor wafers requires sources of particles that form flows of hot (E less than 10 eV) neutral particles of high intensity (1015-1016 cmxe2x88x922sxe2x88x921) at a gas working pressure of less than 10xe2x88x922 Pa in a vacuum camera. However, the sources of neutral chemically active particles were less developed, as compared to the sources of plasma and charged particles.
Mostly developed sources of the kind specifies are sources of atomic hydrogen. The production of hydrogen atoms utilizes several effects as follows:
dissociation of molecules of hydrogen while heating a gas, for example, by laser emission [1],
dissociation by means of high-energy photons, for example, in the UV spectral range [7],
dissociation of molecules on a heated metal surface [8],
dissociated adsorption of molecules followed by electron-stimulated desorption of atoms [9], and
dissociation by electron impact [10].
In the atomic hydrogen sources based on the dissociation of hydrogen on a heated metal surface [11, 12], a dissociator is usually implemented either as a spiral-like tungsten wire heated by the electric current passage therethrough, or as a metal tube heated by electron bombardment. Molecules of hydrogen adsorb on the heated metal surface and dissociate into atoms, which can then leave the surface either as atoms, or, after the recombination, as molecules. Desorption results in the formation of a flow of particles composed of a mixture of atoms and molecules of hydrogen. The effectiveness of such sources at a working pressure of about 10xe2x88x922 Pa is limited to 3% [12] or 15% [13], and, being defined by a sticking coefficient of molecule, does not exceed 25% [14]. Effectiveness of the sources significantly reduces with the increase of the pressure of hydrogen in the source. This prevents formation of intensive flows of atomic hydrogen. The density of atoms"" flow in such a source is typically about 1014 cmxe2x88x922sxe2x88x921. Additionally, this source suffers from incapability of obtaining hot atoms, because of a low temperature of the heated metal surface (xcx9c2000 K).
An atomic hydrogen source based on electron-stimulated desorption of atoms enables formation of atom"" flow with the flow density not exceeding 1014 cmxe2x88x922 Sxe2x88x921 [9, 15] and utilize several sequential physical processes. Initially, the dissociated adsorption of hydrogen molecules takes place on the outer surface of a metal membrane. Then, the atoms diffuse through the membrane, and propagate onto the inner surface of the membrane (in a vacuum). Thereafter, if the atoms are not subjected to any external effect, they will associate into molecules and desorb into vacuum, thereby forming a flow of molecular hydrogen. In order to cause desorption of the atoms from the membrane""s surface, the known effect of stimulated desorption under electron bombardment is used. This results in the formation of a flow composed of hydrogen atoms and molecules. Estimations have shown that the atomic hydrogen source of this kind enables obtaining a flow of hot atoms with the energy of 1 eV [9]. However, the effectiveness of such an atomic hydrogen source is limited by a small cross-section of electron-stimulated desorption of atoms. Hence, in order to obtain an atom flow of 1014 cmxe2x88x922sxe2x88x921, a wide-aperture electron beam with a high current density (more than 10 mAcmxe2x88x922) has to be used. This, in turn, requires using a thermionic emitter of a large surface area heated to a temperature significantly higher than that required in a source utilizing a heated wire. This leads to an increase of the pollution of a semiconductor structure under treatment by tungsten vapor and other products of the desorption process. To provide further growth of the density of an atomic hydrogen flow, the density of the electron current has to be increased even more.
The most effective methods for producing atomic hydrogen are those utilizing dissociation of molecules by an electron impact. Various forms of gas discharge are usually employed in these methods. The techniques of controlling the parameters of gas discharge plasma are well developed, and therefore conditions for effective dissociation of molecules in plasma can be realized. Hot atoms can be obtained by using the dissociation of molecules by electron impact. The dissociation of a molecule into one or two hot atoms is possible in the case, when electron interacting with this molecule has energy higher than the energy required for the molecule dissociation. The transition of the molecule from a highly excited stated, caused by the electron impact, is followed by the molecule dissociation and a partly transform of the redundant energy of electron excitation into kinetic energy of atom(s).
Various sources of this kind have been developed, such as sources of radicals and atomic hydrogen based on radio-frequency discharge [3, 16, 17, 18], microwave discharge under the ECR conditions [19], DC glow discharge [20], and DC arc discharge [21]. Although all these sources are practically capable of creating intensive flows of atoms, they differ from each other in the extent of dissociation of molecules in the discharge. The known atomic hydrogen sources utilizing a gas discharge suffer from the following drawbacks:
An RF discharge based source [18] has a high working pressure, thereby limiting its technological application and impeding the use thereof in super-high-vacuum systems and systems with a relatively low exhaust rate, and consequently, reducing the possibility of obtaining hot atoms (since a high number of interactions between the atoms and molecules in a gas phase results in the reduction of the average energy of atoms). Additionally, this source is characterized by a high energy of ions in the plasma of RF discharge, which may lead to sputtering of the constructional elements of the source and contamination of the surface of a semiconductor structure under treatment, as well as radiation damage and charging of the structure during the ion bombardment thereof.
Sources of the kind utilizing microwave ECR discharge [19] are operable in a wide range of pressure, and are characterized by lower energy of the ions, as compared to that of the RF discharge based sources. Nevertheless, microwave ECR discharge based sources have a complicated construction of both a discharge cell and its power supply source. The need for a discharge cell that meets the specific requirements of geometry, and the need for a strong magnetic field in plasma impede the integration of these sources with standard vacuum equipment. The average energies of Ar, N2 and Cl2 atoms obtained with the ECR based source are about 0.04-0.45 eV [20].
An atomic hydrogen source based on a DC glow discharge is simpler than that of the RF and ECR discharge types. Reference is made to FIG. 1, illustrating this type of source [21]. A discharge cell used therein comprises a hollow cylindrical water-cooled cathode made of molybdenum, and a flat anode made of stainless steel. The cathode and anode are accommodated opposite to each other in the butt-ends of a cylindrical insulator made of aluminum oxide based ceramics. Molecular hydrogen H2 is supplied into the discharge cell through an opening in the butt-end of the hollow cathode. The diameter of the output opening is 2.5 mm, which enables for maintaining the gas pressure drop between the discharge cell and a vacuum chamber. The pressure of hydrogen in the vacuum chamber is maintained at a level of 30 Pa, the pressure inside the discharge cell being about 300 Pa. When direct voltage is supplied to the electrodes, a hollow cathode discharge is ignited in the discharge cell. This discharge is characterized by the growing volt-ampere curve: the growth of the discharge current is followed by an increase of the discharge voltage. When discharge is operating, a flow composed of a mixture of molecular and atomic hydrogen emerges from the cell through an emitting aperture in the flat anode.
Typical values for the discharge current and discharge voltage are, respectively, 0.1 A (too low) and 600V (too high). High voltage leads to the high probability of contamination of a semiconductor structure under treatment by the cathode ion sputtering products, as well as the increased probability of defects formation in near-surface layers of the structure (as a result of ion bombardment thereof). Low values of the discharge current prevent obtaining a high degree of dissociation of molecules of hydrogen in discharge plasma. Additionally, an atomic hydrogen source of this kind can effectively operate only in a narrow range of working pressure values, both in the discharge cell and in the vacuum chamber, and is characterized by a high working pressure.
The electrodes"" geometry used in the source of FIG. 1 provides for a short path of electrons in the volume of the discharge cell, which prevents the effective use of the entire electron energy on the processes of ionization of atoms (molecules) and the dissociation of molecules. Gas dissociation in a discharge cell can be increased by increasing the discharge current. However, in the case of a glow discharge, this causes a growth of discharge voltage and formation of cathode spots, which increases the probability of contamination and damages of the surface of a semiconductor structure.
It has been proposed [22] to overcome the above drawback of the glow discharge based source by using an arc discharge with heated electrode. The arc discharge has a falling volt-ampere characteristic, and the growth of discharge current is followed by a decrease of the discharge voltage. FIG. 2 illustrates the atomic hydrogen source [22] having a discharge cell formed of two electrodes, a pin-like cathode made of thorium-coated tungsten, and a cylindrical anode made of molybdenum, which is water-cooled during operation of the source. The cathode is by its one end supported in water-cooled holder, and by its free end, located in the vicinity of the anode, such that the space between the cathode and anode does not exceed 6.5 mm. A plate made with a conically shaped emitting aperture is located adjacent to the lower butt-end of the anode. The emitting aperture has the following geometry: a length of 1.2 mm, a minimal diameter of 0.4 mm and a solid angle of 30xc2x0. Such an emitting aperture allows for maintaining a large gas pressure drop between a vacuum chamber and the discharge cell. In operation, the discharge cell is placed in a transverse longitudinal magnetic field of 230G. The working pressure inside the discharge cell is (15-25)xc3x97102 Pa, the pressure in the vacuum chamber being 10xe2x88x921-10xe2x88x922 Pa. When a direct voltage is supplied to the electrodes, a glow discharge is first ignited in the discharge cell, which is then, as a result of a specific procedure, transformed into an arc discharge with a self-heating cathode. Transition from the glow discharge into arc discharge causes an increase in the discharge current by 100. Typical values of a discharge current and discharge voltage are, respectively, 15 A and 105V Such regime of discharge operating enables producing an intensive flow of atoms of hydrogen. This source, however, suffers from too high a working pressure of hydrogen in the discharge cell, and too narrow range of working pressure values. Due to the above geometry of electrodes, the following sequence of operations has to be followed when putting the source into operation: creating weak-current glow discharge in the cell at a starting pressure of (20-25)xc3x97102 Pa; slowly increasing the discharge current and pressure of hydrogen to create the abnormal glow discharge; increasing the pressure up to 75xc3x97102 Pa to enable transformation into arc discharge with a heated cathode. Thereafter, the pressure is to be reduced to the working one, (15-25)xc3x97102 Pa, to start the technological treatment. Moreover, the process should be controlled to prevent both the leaps of pressure and leaps of discharge voltage, to thereby avoid discharge variation from the working mode. Incapability of this source for operating at reduced pressure values renders it impossible for use in super-high-vacuum systems and systems with low pumping rate.
According to another technique [24], developed by the inventors of the present application, an atomic hydrogen source based on low pressure arc discharge, schematically illustrated in FIG. 3, comprises a thin-wall hollow cathode 1, a cylindrical anode 2, a flat cathode 3 formed with an emitting aperture 6, and a magnetic field source 4. A magnetic field produced by the magnetic field source provides a Penning discharge in the cell. The hollow cathode partly penetrates into the anode cavity 5, thereby causing creation of a magnetron discharge between the outer surface of the hollow cathode and inner surface of the anode. This magnetron discharge causes heating and creation of thermionic emission from the hollow cathode, thereby causing intensive injection of thermionic electrons into plasma. This source, however, does not provide a sufficiently high density of the output atomic hydrogen flow. Additionally, it is characterized by a short operational time with the same electrodes.
It have been known that effective gas ionization can be obtained by using such forms of gas discharge that utilize crossing electric and magnetic fields (Exc3x97H), i.e., magnetron and Penning discharges, as well as forms of gas discharge utilizing oscillation of electrons between cathodes, i.e. reflective discharge and discharge with hollow cathode [25]. Moreover, such a phenomenon as a plasma jet emerging from the region of a gas discharge into the source surrounding space through a small-diameter aperture has been known from the physics of gas discharge and techniques of plasma and charged particles sources [26]. This phenomenon is used for obtaining plasma flows [27].
There is a need in the art to facilitate the production of atomic particle flow by providing a novel source device for transforming a supplied molecular gas into an intensive flow of atomic particles.
The inventors have found that insufficient density of the atomic hydrogen flow obtained with the earlier source model [24] developed by them is caused by the fact that the hollow self-heating cathode (1 in FIG. 3) is too far from the flat cathode 3. As a result, the plasma density in the zone of emitting aperture is small, that zone of the discharge cell in which hydrogen atoms are mostly generated is at a large distance from the emitting aperture, and atoms on their way to the emitting aperture undergo a large number of collisions with the cold walls of the cell and recombine into molecules. The factor that the source quickly goes out of use is associated with the destroy of that part of the self-heating thin-wall hollow cathode which penetrates into the anode cavity. The hollow cathode is destroyed by ion sputtering, as well as by quick breaking of the electrode material due its over-heating caused by insufficient heat conductance through the thin walls of the hollow cathode.
The inventors take an advantage of the fact that the nature of the mechanisms of dissociation and ionization of molecules are close to each other, and propose obtaining a highly-dissociated gas using the forms of gas discharge utilizing crossing electric and magnetic fields (magnetron and Penning discharges), and forms of gas discharge utilizing oscillation of electrons between cathodes (reflective discharge and discharge with hollow cathode). Additionally, the inventors propose using a plasma jet as an auxiliary source of atomic particles. Experimental results have shown the possibility of realization of these proposals in a new method and device for producing an intensive flow of atoms.
The present invention provides for overcoming the above and other drawbacks of the convention techniques of the kind specified. The source of the present invention can be used for producing atomic hydrogen, nitrogen or oxygen, as well as for producing excited atoms of atomic gas, such as argon or xenon.
According to the present invention, intensive flows of atoms of molecular gases and excited atoms of atomic gases are obtained from plasma of gas discharge. The present invention utilizes several original approaches for solving the problem of optimizing the parameters of the plasma and geometry of the discharge cell""s electrodes, as well as for satisfying the requirements of the sources of neutral particles.
There is thus provided according to one aspect of the present invention, a method of producing an intensive flow of atoms from an input flow of a molecular gas with a source comprising a discharge cell connectable to a direct current source and defining at least one emitting aperture through which the flow is output from the cell, the method utilizing ignition of a gas discharge in said discharge cell and dissociation of the gas molecules by electron impact, and comprising:
providing ignition of the gas discharge of a complex type composed of a main discharge and two auxiliary discharges of different types ignited in substantially coinciding zones of the discharge cell, wherein
said main discharge is an arc Penning discharge ignited in a zone of the vicinity of said at least one emitting aperture,
the first auxiliary discharge is a magnetron discharge with heated cathode, and
the second auxiliary discharge is one of the following: a Penning discharge, and a Penning discharge with hollow cathode,
the dissociation of the gas molecules being thereby carried out in said complex discharge and resulting in creation of the flow of hot and thermally atoms.
The hot atoms are atoms with the energy of about 0.1-10 eV, and the thermally atoms are those with the energy less than 0.1 eV.
According to another aspect of the present invention, there is provided a source device for producing an intensive flow of atomic or excited particles, the device being connectable to a direct current source and comprising an electrodes"" arrangement and a magnetic field source, wherein the electrodes"" arrangement comprises a cylindrical anode and a multiple-electrode cathode which are axially aligned and define an inter-electrode space for a longitudinal magnetic field region, wherein
the multiple-electrode cathode comprises a first elongated self-heating electrode, a second flat reflective electrode in which at least one opening forming at least one emitting aperture is made, and a third reflective electrode the first electrode being electrically connected to the third electrode, when the device is put in operation;
the first self-heating elongated electrode is axially aligned with the cylindrical anode and penetrates into the anode cavity at a predetermined distance;
a butt-end of the first electrode located inside the anode cavity, a part of the surface of the second electrode opposite a butt-end of the first electrode and the
cylindrical anode form a cell of a main arc Penning discharge ignitable in at least one zone in the vicinity of said at least one emitting aperture;
the first electrode and the cylindrical anode form a cell of a first auxiliary discharge, which is a magnetron discharge with heated cathode; and
the second and third reflective electrodes and the cylindrical anode form a cell of a second auxiliary discharge, which is one of the following: a Penning discharge, and a Penning discharge with hollow cathode.
The invented method for obtaining intensive flows of atoms provides significant prevalence of the rate of generation of atomic particles in plasma of gas discharge by means of molecules dissociation by electron impact (bombardment), over the rate recombination of atoms into molecules. The flow of atomic particles is separated from plasma of a combined form of arc discharge with heated cathode. A region of dense plasma is created in a discharge cell, wherein this region is characterized by a high concentration of fast, as well as thermionic electrons, emitted from the surface of the heated cathode. The dense plasma region is located in the vicinity of an emitting aperture in a flat cathode, through which atoms forming the flow are output. The probability of atoms"" recombination into molecules on the surface of the emitting aperture can be reduced by making the emitting aperture in the foil of a refracting metal. Such a refractory metals may be Re, W, Mo, or WRe alloy. This results in that the surface of the cathode in the vicinity of the emitting aperture is heated up to a high temperature by ion bombardment. The main Penning discharge with self-heated electrode (cathode) is used to form the dense plasma region. To maintain the main discharge and effective heating of the self-heated electrode, two auxiliary discharged with somewhat less dense plasma are used: the magnetron discharge with heated cathode and Penning discharge with hollow cathode (or Penning discharge without hollow cathode). The auxiliary discharges operate at a certain distance from the emitting aperture. A molecule gas is input into the discharge cell from a side opposite to the emitting aperture, and enters the regions (zones) of the auxiliary discharges, where a part of molecules dissociate into atoms. Most of these atoms recombine on cold walls of the electrodes of the discharge cell, and an insignificant part of the atoms, that has not undergone a large number of interactions with the cold walls, is output into the flow of atoms. The entire gas that has passes the zones of the auxiliary discharges, enters the zone of the main discharge, where the most of the remaining molecules dissociate into atoms, which are output into the flow substantially without losses associated with recombination.
Thus, the main idea of the present invention consists of creating the region of dense plasma in the vicinity with a high concentration of fast electrons in a small xe2x80x9cpoint-likexe2x80x9d volume in the vicinity of the emitting aperture. In this region, conditions for a high rate molecule"" dissociation and a small rate atom"" recombination are provided, and the entire gas is pumped through this region.
To even more increase the effectiveness of the source (i.e., increase of the degree of gas dissociation), additional dissociation of molecules can be provided in the region of atoms"" flow emerged from the source. To this end, a plasma jet can be formed propagating into vacuum through the emitting aperture. Fast thermionic electrons coming from the self-heated electrode oscillate in the plasma jet and produce effective dissociation of the remaining molecules. Gas dissociation in the region of the main discharge and in the plasma jet by the fast thermionic electrons leads to the increase of the part of hot atoms in the entire flow of particles emerging from the source. By supplying an atomic gas into the discharge cell, flows of excited atoms can be obtained.
The present invention can be used for treatment of a semiconductor structure aimed at modifying the properties of surface and/or near-surface layers of the structure. For example, this can be used for cleaning the surface of a semiconductor structure from oxides, organic, metal and other contaminations, as well as for residual photoresist removal; hydrogenation of near-surface layers of a semiconductor structure; assisting in thin-film deposition processes; treatment of semiconductor structures based on mono-crystal, poly-crystal and amorphic substrates and/or layers fabricated from elementary semiconductors, semiconductor compounds and/or solid solutions.